Alloy steel and method



United States Patent 3,123,468 ALLOY STEEL AND METHOD Harry Tanczyn, Baltimore, Md., assignor to Armco Steel Corporation, a corporation of Ohio No Drawing. Filed June 28, 1960, Ser. No. 39,221 15 Claims. (Cl. 75-428) My invention is concerned with the chromium-nickel alloy I steels and more particularly the heat-hardenablc chromium-nickel stainless steels.

One of the objects of my invention is to provide a chromium-nickel-molybdenum alloy steel which readily lends itself to forming, shaping and other fabrication in the annealed condition and which is capable of being hardened from the annealed condition with simple lowtemperature ageing treatment to achieve high strength and hardness at both low temperatures and high temperatures.

A further object is to provide a method for conditioning alloy steels of the character described wherein, following shaping, forming or other fabrication, the metal is thereafter quickly, economically and reliably hardened and strengthened by Way of a simple and comparatively low-temperature ageing treatment of relatively short duration.

Other objects of my invention and advantages of the same will be obvious in part and in part more fully pointed out during the course of the following disclosure.

My invention therefore resides in the combination of elements, in the compositions of materials, in the several operational steps, and in the relation of each of the same to one or more of the others as described herein, the scope of the application of which is indicated in the claims at the end of this specification.

'In order to better understand my invention, it may be noted at this point that the invention is directed primarily to an improvement in the chromium-nickel alloy steels. Of this group of steels the well-known austenitic chromium-nickel grades are defined as having a chromium content ranging from about to 35%. The nickel content may range from something above 0% on up: to 25% or more, nickel being included for the dual purpose of improving the stainless qualities of the steel and controlling the structure of the steel and with this the softness, ductility, and magnetic qualities thereof. With nickel high the steel is essentially austenitic. Conversely, with nickel low relative to chromium, the metal is either typically martensitic or ferritic, depending upon the carbon content.

In the known-chromium-nickel alloy steels various alloying metals may be included in small amounts to im part special properties. Typical of these are aluminum, copper, molybdenum, manganese, cobalt, silicon, sulphur, phosphorus, tungsten, vanadium, titanium, columbium and the like. The remainder of the composition is substantially all iron. Usually the steels are of low carbon content, say about 0.03% to a maximum of about 0.20%. For special purposes, however, the carbon content may be substantially higher.

The 18-8 chromium-nickel stainless steel, of course, is characteristically austenitic. And such an austenitic steel is non-magnetic. It hardcns upon working by known forming, working or machining operations at room temperature. Moreover, it work-hardens as by riveting, or like joining treatment. Such a steel, however, with its high values of both chromium and nickel, is virtually immune to phase-transformation when subjected to heating and a subsequent quench. The soft austenitic condition, characteristic of the steel, is retained throughout heat-treatment; the steel is not hardenable by heat-treating methods to improve strength and hardness. In some respects this is viewed as a disadvantage.

A further disadvantage of the austenitic chromiumnickel stainless steels is that upon rolling, drawing, or other Work conducted in a single direction, the workhardening effect essentially is along the direction of working and the mechanical properties transverse of the direction of working are not developed to the same extent as those in the working direction. The disparity is particularly noted in compression. In many applications a difference in the longitudinal and transverse compressive strengths is not acceptable.

Moreover, in the austenitic chromium-nickel stainless steels the tendency to harden in working requires frequent interruption of the Working operation to relieve the strains induced by working, as by subjecting the metal to intenrnediate anneal. And frequently, such intermediate anneal is attended by a loss of the nice dimensionality imparted the products, as through light scaling, or perhaps distortion.

As contrasted with the Well-known austenitic chromium-nickel stainless steels briefly discussed above, there are the precipitation-hardenable chromium-nickel stainless steels, that is, the chrornium-nickel stainless steels which are hardcnable by heat-tre atment. Typical of these are the Armco 17-7PH (about 17% chromium, 7% nickel, 1% aluminum, and remainder iron), the Armco 174PH (about 17% chromium, 4% nickel, 4% copper, and remainder iron) and the Arm-co- PH15-7M0 (about 15% chromium, 7% nickel, 2% molybdenum, 1% aluminum, and remainder iron).

In their annealed conditions, these steels possess good working and forming characteristics. And in their hardened conditions the steels display many highly desirable physical properties. Illustratively, the steels are hard, durable and of high tensile strengths.

In the past various efforts have been made, as by varying composition and heat-treatment, to accomplish the requisite phase-transformation at reduced temperatures. But, in general, the results leave much to be desired, even where there are employed closely controlled percentages of added hardening agents such as columbium, titanium, vanadium and tungsten and, where time and temperature of treatment have been tried in numerous combinations. And while the precipitation-hardened products possess admirable strength at room temperatures, both hardness and strength rapidly diminish with use at temperatures above about 900 F.

One of the objects of my invention, therefore, is to provide a chromium-nickel alloy steel which, in the annealed condition, lends itself to fabrication into a variety of intricate and complex shapes and which readily lends itself to hardening in substantial absence of scaling and physical distortion, in simple operation under conditions 3 of comparatively low hardening temperatures of relatively short duration, to achieve strength of high value both at room temperatures and at elevated temperatures as well.

Referring now more particularly to the practice of my invention, I find that with a proper combination of molybdenum, chromium and nickel, I achieve an al oy steel which is subsequently hardenable by heat-treatment from a soluble solution-treated (annealed) condition. And that this hardening is had by way of a single-step hardening operation of comparatively short duration conducted at relatively low temperatures. Where desired, it also can be hardened by double-treatment, this from an even more ductile annealed condition.

The hardened steel is strong and hard both at room temperatures and at high temperatures. The mechanical properties of the steel compare favorably with known and available precipitation-hardened steels.

The alloy steel of my invention essentially consists of about to chromium, about 3.5% to 12% nickel, about 4% to 12% molybdenum, with remainder substantially all iron. Carbon is included in amounts of about 0.005% to 0.15% and nitrogen in amounts up to 150%. Manganese also may be present in amounts up to 2.50%. Phosphorus is present in amounts up to 0.05% maximum, sulphur up to 0.05% maximum, and silicon up to about 2.00%. Where desired, columbium and tantalum are employed in total amount up to about .75 At times, for specific purposes, aluminum may be used in amounts up to .40% to control delta ferrite. Copper may be present in amounts up to 3.0%, vanadium up to about 1%, titanium and/ or zirconium up to about 0.50%, and boron up to 0.010%. In some instances, where extreme hardness is desired and some calculated sacrifice in duetility of resulting steel is permissible, tungsten may be substituted for molybdenum, in amounts up to about 7% tungsten. As more particularly noted hereinafter, the ratio between the sum of the chromium and molybdenum contents of the steel and the nickel content does not exceed about 4.5.

For the single-treatment steel of my invention I employ about 005% to .12% carbon, 5% to 14% chromium, about 4% to 12% nickel, about 4% to 12% molybdenum, up to .150% nitrogen, and remainder substantially all iron. The double-treatment steel essentially consists of about .01% to .15% carbon, 7% to 15% chromium, 3.5% to 10% nickel, 4% to 12% molybdenum, up to .150% nitrogen, and remainder substantially all iron.

In the steel of my invention the percentage of molybdenum employed and its relation to the chromium and nickel contents thereof are both critical. For if the steel is either too low in chromium or too high in molybdenum, practical difficulties are encountered. I find that with a chromium content near the lower part of the given range molybdenum may be somewhat higher than would otherwise be permissible; while if the chromium content a proaches the upper limit of permissible range, then the molybdenum content must be correspondingly diminished.

The steel of my invention displays requisite softness and ductility following a preliminary solution-treatment, that is, an anneal, as by heating for about one-half hour at temperatures of about 1600 F. to 2000 F. And in this condition the steel can be worked with comparative ease to close dimensional tolerances and required intricacy of configuration. The products readily lend themselves to hardening by a simple ageing treatment conducted at co-mparately low temperatures and enduring for relatively short periods. of time, all as more particularly described hereinafter. And the hardened products thereby acquire enhanced qualities of hardness and strength. These qualities are retained during prolonged operation at both room temperatures and elevated temperatures.

A preferred chromium-nickel-molybdenum alloy steel according to my invention which is hardenable by singletreatment analyzes about 9% to 12% chromium, about 5% to 8% molybdenum, with the sum of the chromium and molybdenum contents about 15% to 19%, about 5% to 8% nickel, the nickel content being inversely proportioned with respect to the sum of the chromium and molybdenum contents, carbon .05% to 0.12%, nitrogen 005% to .150%, manganese .01% to 2.0%, phosphorus up to about 0.05% maximum, sulphur up to about 0.05% maximum, silicon about .10% to 2.0%, remainder substantially all iron. This steel is stainless.

A preferred stainless steel which is hardenable by double-treatment analyzes about 7% to 15% chromium, about 3.5% to 10% nickel, about 4% to 12.0% molybdenum, 02% to .15% carbon, 005% to .150% nitrogen, and remainder substantially all iron.

In order to bring the single-treatment steel of my invention into a soft condition which is suitable for fabricating operations, I subject the metal to a solution-treatment, that is, an anneal, at temperatures ranging from about 1600 to 2000 F. I find that these relatively high annealing temperatures effectively serve to place the metal in an austenitie and molybdenum-soluble condition. I further find that the steel when cooled as by quenching in air, oil or water, is transformed to a martensitic condition, with molybdenum in solution. Although the length of time at annealing treatment is not too critical 1 find that at an annealing temperature of 2000 F. a treatment enduring for approximately one-half an hour is entirely satisfactory. Lower temperatures, of course, would require longer times. The annealing treatment is conducted in any heat-treating furnace suitable for the purpose.

In its annealed and quenched condition, my singletreatment steel displays a structure which is basically martensitic, a soft martensite. This is clearly discernible under the microscope. Moreover, it is usually free of delta-ferrite. It is ductile and possesses good qualities of directionality, together with hardness of about Rockwell C28-38.

The double-treatment steel of my invention preferably is solution-treated at a temperature of 1800 to 2100 F. and cooled to room temperature. With this treatment the steel is in an austenitic condition. The hardness is less than Rockwell B100.

The steel, following solution-anneal, is readily formed and fabricated as by punching, bending, stretching, shrinking, or the like, or by drilling, cutting, threading, and so forth; particularly is this true of the double-treatment steel where treated at the higher solution-anneal temperatures. The steel is successfully and readily brazed and soldered. Or it can be welded in accordance with known welding operations.

Upon conclusion of the fabrication operations I harden the single-treatment steel, preferably by an ageing treatment conducted at relatively low temperature but for a short period of time. More particularly, I subject the steel products to an ageing temperature of approximately 850 to 1400 F. for a period of approximately one hour. Following the ageing treatment, I cool the steel, as in air or water. This single-step, low-temperature ageing treatment, of relatively short duration, apparently brings about a precipitation of a molybdenum-rich phase. It may be supposed that it is this precipitated phase, rich in molybdenum and distributed uniformly throughout the steel to which may be attributed the improved mechanical properties of hardness and strength. Unlike many known precipitation-hardened alloys, the steel of my invention retains these properties even following prolonged operation at high temperatures. This I attribute to the stability of the molybdenum-rich phase present in the hardened condition of the steel.

The double-treatment steel, following fabrication, preferably is subjected to transformation by reheating at a temperature of about 1200" to 1700" F. and cooling to a temperature of 90 to -320 F. The times of heating and of cooling are not particularly critical although the heating usually is for a period of an hour or more and the cold treatment is for a period up to 24 hours or longer.

tion are given in Table II(a) below, the mechanical prop erties of the samples in the hardened condition being given in Table II(b):

TABLE II(a) The if f g g fd is: f g 5 Room Temperature Mechanical Properties of the Steels may fans Olme y co ms ea 0 y e of Table I in the Annealed Condition reheating and cold-treatment noted.

Following transformation, I harden the steel by again Heat UITZS, 02% 37's" Per'cent Percent Rockwell reheating, this at a temperature of 800 to 1200 F. for p- -ip.s.1. 101.1112 Red. Area "0 Hard. periods up to 12 hours. The hardened steel is possessed 10 R24i9 100, 000 105, 000 12 43 35 of excellent mechanicakproperties. Rum 162 000 110,000 12 48 36 As more specifically illustrative of the practice of my 1gg% 000 44 2g 4,000 52 invention, one preferred single treatment composition B24234" 164,000 1127000 12 46 37 analyzes approximately 12% chromium, 6% nickel, 6% R2425--- 143, 000 97,000 13 54 32 molybdenum, and remainder substantially all non. A 15 R2427 128,000 78,000 16 58 28 further preferred composition analyzes approximately 1 103 og 5s 27 0 0 19 5 27 10% chromium, 7% nickel, 6% molybdenum, and re 135,800 76,100 20 49 28 mainder substantially all non. Another analyzes about R2557 148, 700 ,900 13 54 23 9% chromium, 8% nickel, 6% molybdenum, and refigggg: 38g gg'ggg g 2% 2g mainder substantially all iron. Others analyze about 20 R2501 157,200 114,500 10 00 31 10% chromium, 7% nickel, 7% molybdenum, and re- 15O'OOO 1001000 17 5O 30 mainder substantially all non; about 12% chrom1um, 120003 F for%h,.lzmd oflquenchw 5% nickel, 6% molybdenum, and remainder substantially TABLE 11(1)) an iron; and about 12% chromium 5% nickel 7% Room Tem erature Mechanical Pro erties of the Steels molybdenum, and remainder substantially all iron. Also 25 ofpTable I in the Hardenedlg n 1 about 11% chromium, 5% nickel, 6% molybdenum, and l w remainder substantially all iron; and about 11% chromium, 5% nickel, 7% molybdenum, and remainder subig' g g Ef igy, gg fig 3%?52 stantially all iron. The alloy steels of my invention with less than about 8% chromium are not considered to be 32mm 230,000 165,000 12 43 43 stainless steels because I find that, where the chromium 3 2 2 .000 13 47 6 content is less than about 8%, there is loss of passivity. 5 3 1338 @1883 a is 1% For the several preferred single treatment steels it Will be gig?- ggg 1%.000 1? 38 Z? seen that the sum of the chromium and molybdenum 3: 5 1 1888 15 2 46 contents ranges from about 15% to about 19%, with the 0 0.000 17 52 46 k l t t 1 f o b t 87 R2552 147, 300 07,300 21 59 32 111C fi conben S scoyirespon ingy rafngir zlgl rhrrn a On 1 0 25 Pg 1 00 110 909 18 52 24 n to a on ivin r tio 255 5, 00 118,300 21 59 7 devil bd t g or e c om i pus R2557..- 181, 900 147,200 18 50 35 mo y enum contenltls o a out .9 to 3.8. Actua y, as 23? 388 161,900 5 g more fully appears ereinafter (Table I), this ratio may 40 R2561 213, 400 185,900 16 53 43 range from about 1.7 to 4.5. R2562-.. 217,100 131,300 15 43 45 The analyses of a number of chromium-nickel-molybdenum single-treatment stainless steels according to my 2000 F. for %hour and oil quenched, +1050 F. for 1 hour and air invention are given in Table I below: owed- TABLE I Specific Examples of Single-Treatment Cr-Ni-Mo Stainless Steels Heat No. 0 MN* 1 s* 31* Or N1 M0 Cb Cr+Mo l I *Mn, P, S and Si checked on R2419 and R2552. Additions of these elements were the same for every other heat in this group.

It is noted that the sum of the chromium and molyb- Now as to the double-treatment steels of my invention,

denum contents for the specific examples given above a preferred steel analyzes approximately 11% chromium,

ranges from a low of 14.82 (heat No. R2422) to a high of 19.68 (heat No. R2552) and that the ratio of Cr-i-Mo to Ni for these various examples ranges from a minimum of 1.78 (heat No. R2426) to a maximum of 4.72 (heat No. R2552).

Specimens from the several steels identified in Table I were first annealed at 2000 F. for /2 hr. and oil quenched. Following this they were heated at 1050 F. for 1 hour and air-cooled. The room temperature me 6% nickel, 8% molybdenum, and remainder substantially all iron. Another analyzes about 12% chromium, 6% nickel, 7% molybdenum, and remainder substantially all iron. A further steel analyzes about 13% chromium, 6% nickel, 6% molybdenum, and remainder substantially all iron. In these steels the carbon content amounts to about 08%, the manganese about .60%, the phosphorus about .010%, the sulphur about 010%, the silicon .40%, and the nitrogen .025%. For these prechanical properties of the samples in the annealed condiferred steels, the sum of the chromium and molybdenum contents is about 19%, the nickel about 6%, and the ratio of the sum of the chromium and molybdenum contents to the nickel content is about 3.2.

The analyses of several double-treatment stainless steels according to my invention are set forth in the Table III below:

TABLE III Specific Examples of Double-Treatment Cr-Ni-Mo Stainless Steels 11000120 0 M11 P s 81 Cr Ni Mo N Cb Cr-I-Mo 1 Here it is seen that the sum of the chromium and nickel contents for the specific examples given ranges from 16.85 (heat No. R2643) to 18.94 (heat No. R2641), and that the ratio of the Cr-I-Mo to the Ni, for these examples, ranges from a minimum of 2.41 (heat No. R2643) to a maximum of 3.15 (heat N0. R2641).

In the annealed condition (2000 F. for 30 minutes and air-cooled) the hardness of the examples given in Table III does not exceed Rockwell B97.

The mechanical properties of the steels of Table III after hardening by double-treatment, specifically heating at 14GO F. for 1 /2 hours and air-cooling to 75 F. to effect transformation, then reheating at 1050 F. for 2 hours and air-cooling, are given in Table IV (a) below:

TABLE 1 7 0 [Anneal 2000 F. hr.water quench; plus 1,400 90 mins.-eo0l to 75 F.; plL'S reheat 1,050 F. 2 hoursa1r cool] Heat Ult. Ten. 0.2% Y S Percent Percent Rockwell No. str., p.s.i. p.s.i Elong. in 2 Red. Area Hardness R 2640- 195, 000 170, 000 12 44 C42 R2641- 202, 000 165, 000 13 46 C43 R2643 204, 000 167, 000 12 42 C43 R2644. 208, 000 185, 000 10 38 C44 112645- 220, 000 181, 000 11 41) C44 R2646. 209, 000 178, 000 12 40 C45 R2047 214, 000 189, 000 11 38 C40 R2648." 212,000 182, 000 14 O46 R2649. 217,000 187, 000 12 36 047 Similar mechanical properties are had when the steel is heated at 1700 F. for minutes, cooled to room temperature, and then refrigerated at 100 F. for 8 hours ot effect transformation, then reheated to 1050 F. for 2 hours and air-cooled to harden. These properties are given in Table IV(b) below:

TABLE IV {12) [Annealed 2,000 F. mins.water quench; plus 1,700 F. 20 mins. cool to room temperature and refrigerate at 100 F. 8 hrs.raise to room temperature; plus reheat to 1,050 F. 2 hrs.a1r cool] By comparing the analyses of Table III with those of Table I it will be seen that many may be viewed as both single-treatment and double-treatment steels. And the choice of heat-treatment, whether single or double, is

emperature to assure a fully austenitic structure, and hardening had by transformation treatment followed by ageing. On the other hand, where the amount of deformation is insubstantial and maximum ductility in the solution-treated condition is not required, then a solution-treatment at a lower temperature is adequate and hardening is achieved by simple ageing treatment of the solution-treated steel without necessity for the separate transformation step; actually, the solution-treatment at the lower temperature range and cooling effects partial transformation, at least sufiicient to assure hardening by the subsequent ageing step.

The steels of my invention not only are suited to duty at room temperature but, in addition, they are well adapted to service at elevated temperatures.

Accordingly, it will be seen that I provide in my invention a steel and method of heat-treating the same, in which the many objects and advantages hereinbefore set forth are successfully achieved. My steel displays markedly superior mechanical properties, particularly strength and hardness, and is well suited to duty at room temperatures, as Well as at high temperatures. In my steel there is had a good resistance to corrosion, even when subjected to reducing acids. And, it is to be noted, the improvement in corrosion resistance is without sacrifice of the precipitation-hardenable qualities of the steel. Upon age-hardening, the steel achieves considerable resistance to Wear, abrasion and galling.

The steels of my invention lend themselves admirably to the production of wrought products as well as castings. The metal in the solution-treated or annealed condition can be machined, welded and formed with comparative ease.

Inasmuch as many embodiments may be made of my invention as described herein and inasmuch as many changes in the present embodiments will suggest themselves to those skilled in the art, it will be understood that i desire the foregoing disclosure to be considered as simply illustrative, and not as a limitation.

I claim as my invention:

1. Precipitation-hardenable chromium-nickel-molybdenum alloy steel essentially consisting of about 5% to 14% chromium, 4% to 12% nickel, 4% to 12% molybdenum, with the ratio between the sum of the chromium and molybdenum contents and the nickel content ranging from about 1.7 to 4.7 and the remainder essentially iron.

2. Precipitation-hardenable chromium-nickel-molybdenum alloy steel essentially consisting of about 7% to 15% chromium, 3.5% to 19% nickel, 4% to 12% molybdenum, with the ratio between the sum of the chromium and molybdenum contents and the nickel content ranging from about 2.4 to 3.2 and the remainder essentially iron.

3. Precipitation-hardenable stainless steel essentially consisting of approximately: 9% to 12% chromium, to 8% nickel, 5% to 8% molybdenum, with the sum of the chromium and molybdenum contents about 15% to about 19%, and the remainder essentially iron.

4. Precipitation-hardenable stainless steel essentially consisting of approximately: 9% to 12% chromium, 5% to 8% molybdenum with the sum of the chromium and molybdenum contents about 15 to about 19%, 5% to 8% nickel with the nickel content being inversely proportioned to the sum of the chromium and molybdenum contents, and the remainder essentially iron.

5. Precipitation-hardenable stainless steel essentially consisting of approximately: 9% chromium, 8% nickel, 6% molybdenum, and remainder essentially iron.

6. Precipitation-hardenable stainless steel essentially consisting of approximately: 10% chromium, 7% nickel, 7% molybdenum, and remainder essentially iron.

7. Precipitation-hardenable stainless steel essentially consisting of approximately: 11% chromium, 5% nickel, 6% molybdenum, and remainder essentially iron.

8. Precipitation-hardenable stainless steel essentially consisting of approximately: 11% chromium, 5% nickel, 7% molybdenum, and remainder essentially iron.

9. Precipitation-hardenable stainless steel essentially consisting of approximately: 11% chromium, 7% nickel, 6% molybdenum, and remainder essentially iron.

10. Precipitation-hardenable stainless steel essentially consisting of approximately: 12% chromium, 5% nickel, 6% molybdenum, and remainder essentially iron.

11. Precipitation-hardenable stainless steel essentially consisting of approximately: 12% chromium, 5% nickel, 7% molybdenum, and remainder essentially iron.

12. Precipitation-hardenable stainless steel essentially consisting of approximately: 12% chromium, 6% nickel, 6% molybdenum, and remainder essentially iron.

13. Precipitation-hardenable stainless steel essentially consisting of about 11% chromium, 6% nickel, 8% molybdenum, and remainder essentially iron.

14. Precipitation-hardenable stainless steel essentially consisting of about 12% chromium, 6% nickel, 7% molybdenum, and remainder essentially iron.

15. Precipitation-hardenable stainless steel essentially consisting of about 13% chromium, 6% nickel, 6% molybdenum, and remainder essentially iron.

References (Iited in the file of this patent UNITED STATES PATENTS 2,185,996 Hatfield Jan. 9, 1940 2,482,097 Clark Sept. 20, 1949 2,505,763 Goller May 2, 1950 2,695,229 Sheridan et a1 Nov. 23, 1954 2,797,993 Tanczyn July 2, 1957 2,850,380 Clarke Sept. 2, 1958 2,875,042 Kegerise et a1 Feb. 24, 1959 2,958,618 Allen Nov. 1, 1960 

1. RECIPITATION-HARDENABLE CHROMIUM-NICKEL-MOLYBDENUM ALLOY STEEL ESSENTIALLY CONSISTING OF ABOUT % TO 14% CHROMIUM, 4% TO 12% NICKEL, 4% TO 12% MOLYBDENUM, WITH THE RATIO BETWEEN THE SUM OF THE CHROMIUM AND MOLYBDENUM CONTENTS AND THE NICKEL CONTENT RANGING FROM ABOUT 1.7 TO 4.7 AND THE REMAINDER ESSENTIALLY IRON. 